1. Field of the Invention
The present invention relates to a method for removing inorganic foreign matters strongly attached to the surface of a substrate made of silicon or glass to be used as a silicon wafer, a photomask substrate or a photomask blank, the surface of a glass substrate for a template to be used for nanoimprinting, the surface of a substrate made of glass to be used as a substrate for various display devices, or the surface of an inorganic coating formed on the surface of such a substrate.
2. Discussion of Background
Heretofore, in a process for producing an integrated circuit, photolithography process which comprises reduction projection of a circuit pattern on a mask onto a wafer by using an exposure device to form an integrated circuit on the wafer has been widely utilized. Along with high integration and high functionality of integrated circuits, microsizing of an integrated circuit is in progress, and an exposure device is required to form an image of a circuit pattern with very small dimensions on a wafer with a long focal depth, and blue shift of the exposure light source is in progress. The exposure light source has been advanced from the conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) or KrF excimer laser (wavelength: 248 nm), and now an ArF excimer laser (wavelength: 193 nm) is being used. Further, in order to be prepared for a future integrated circuit where the line width of a circuit pattern will be at most 50 nm, use of a F2 laser (wavelength: 157 nm) or EUV light (extreme ultraviolet light: wavelength: about 13 nm) as the exposure light source is promising.
Along with microsizing of the dimensions of an integrated circuit, the allowable size of foreign matters on a mask or a wafer has been increasingly small. For example, according to ITRS (International Technology Roadmap for Semiconductors), 2005, foreign matters allowable on a wafer to be used for production of a 32 nm half-pitch DRAM (random access memory) or in a resist coated on the wafer, are required to be such that “the density of foreign matters with a size of 20 nm is at most 0.01 foreign matter/cm2”, and in the case of a mask, it is required that no foreign matters with sizes of 26 nm or lager are present on the mask. Further, as a method of forming very small irregular patterns, nanoimprinting has been proposed and actively studied in addition to lithography. Since a 1x mask (template) is employed in nanoimprinting, it is required to reduce foreign matters with smaller sizes to the similar density, as compared with the requirement regarding foreign matters on the mask substrate for lithography. The required size of the foreign matters is smaller by about 30 nm than that in the case of a 80 nm half-pitch DRAM, and thus a method for effectively removing such very small foreign matters from the surface of a substrate made of silicon or glass to be used as a wafer or a mask, is required.
Further, in the case of a mask, the surface roughness is preferably as small as possible for the purpose of suppressing light scattering on the surface along with blue shift of the exposure light source. For example, in the case of a substrate for a reflective mask to be used for EUV lithography, a surface roughness (RMS) of at most 1.5 Å is required. Therefore, in the case of a substrate to be used as a mask, it is required to effectively remove the above very small foreign matters while the increase of the surface roughness is suppressed as far as possible.
As a method for removing foreign matters from a substrate made of silicon or glass, so-called lift-off process is commonly employed, i.e. a method of exposing the substrate surface to a chemical solution such as an acid or an alkali to carry out wet etching of the substrate surface in a very small amount to remove the foreign matters. For example, as proposed in Patent Document 1, the surface of a quartz glass substrate is exposed to hydrofluoric acid to carry out wet etching of the substrate surface, so that the distance between the substrate surface and foreign matters (e.g. inorganic foreign matters derived from colloidal silica glass or an alumina powder used for abrasion) attached to the substrate surface is extended to such an extent (at least about 5 to 10 Å) that the adhesion force (mainly Van der Waals force) will no more work out, whereby the foreign matters can be removed from the substrate surface. After the exposure to such a chemical solution, the substrate surface is rinsed with e.g. deionized water to replace the chemical solution with the deionized water, followed by drying. Further, in the case of a substrate to be used as a silicon wafer, Patent Document 2 for example proposes to remove foreign matters attached to the wafer surface by lift-off process in the same manner as above, by exposing the surface of the substrate to an aqueous solution containing hydrogen fluoride and hydrogen peroxide to oxidize silicon on the surface into silicon oxide utilizing oxidizing power of a hydrogen peroxide solution, and wet etching the resulting silicon oxide with hydrofluoric acid.
Here, in view of removal of foreign matters attached to a substrate by lift-off process, presence of electrical repulsive force between the foreign matters and the substrate is preferred. Namely, the foreign matters and the substrate surface preferably have the surface potential of the same sign. Further, in a case where the foreign matters and the substrate surface have the surface potential of the same sign, the absolute value of the difference in the surface potential between the foreign matters and the substrate is preferably as large as possible, whereby greater electrical repulsive force will be obtained.
The surface potential of most of substances (such as SiO2, Si3N4 and organic substances) which can be foreign matters present on a substrate material or a substrate greatly depends on the pH of the liquid to which the surface is exposed. The surface potential is substantially 0 or slightly positive in the presence of a strong acid such as hydrofluoric acid i.e. in a region of pH=3 to 4, and the surface potential is negative in a weakly-acidic to alkaline region (3 to 4<pH). However, there are some exceptions. For example, the surface potential of Si has not have great pH dependence, and the surface potential is stably negative in a wide region of from acidic to alkaline (pH=3 to 11).
However, in a case where an acidic solution having a pH<7 is used as a chemical solution for the lift-off process, pH of the chemical solution is close to the equipotential, the surface potential of the substrate and the foreign matters is substantially 0 and accordingly substantially no electrical repulsive force is obtained. Thus, not only removal of the foreign matters from the substrate is difficult, but also the foreign matters suspended in the chemical solution, after once removed from the substrate, are likely to be attached to the substrate again.
Patent Document 1: JP-A-2002-131889
Patent Document 2: JP-A-3-120719